Η4-diene Complexes Research Articles

Oxidation of Ruthenium(II) to Ruthenium(IV) η4-Diene Complexes: Swing Mechanism and Diene−Allyl Conversion

This paper reports on the synthesis and reactivity of Ru(IV) η4-diene complexes of the type Ru(η5-C5Me5)(η4-diene)Br2+ obtained by oxidative bromine additions to Ru(II) η4-diene complexes. Mechanistic details are derived from the reaction products varying with the oxidizing agent and the leaving ligand and are backed up by extended Huckel molecular orbital calculations. The diene fragment in Ru(IV) is prone to gauche deformation and is therefore extremely susceptible to nucleophilic attack of even weak anionic bases generating the corresponding Ru(IV) η3-allyl complexes. Accordingly, Ru(IV) η4-diene complexes are stabilized either by using 2,3-disubstituted diene ligands in which case gauche deformation is highly unfavorable or else by excluding nucleophilic agents. For the latter method, two convenient routes to affording Ru(IV) η4-diene complexes are (i) the reaction of Br+CF3SO3- with Ru(η5-C5Me5)(η4-diene)Br and (ii) the reaction of dibromine with labile Ru(η5-C5Me5)(η4-diene)(CF3SO3). The conversion ...

Synthesis and Characterization of Dibromo-Containing Ruthenium(IV) η3-Allyl and Ruthenium(IV) η4-Diene Complexes. Formation of [Ru(η5-C5Me5)Br3]- and [Ru(η5-C5Me5)Br3]2

The reaction of Br2 with Ru(η5-C5H5)(η4-diene)Br (diene = 1,3-butadiene (1a), 2-methyl-1,3-butadiene (1b), 1,3-hexadiene (1c)), Ru(η5-C5Me5)(η4-diene)Br (diene = 1,3-butadiene (2a), 2-methyl-1,3-butadiene (2b), 3-methyl-1,3-pentadiene (2c), 1,3-hexadiene (2d), 1-methoxy-1,3-butadiene (2e), 2,4-hexadiene (2f), phenyl-1,3-pentadiene (2g), diphenyl-1,3-butadiene (2h), 1,3-cyclohexadiene (2i), 2,3-dimethoxy-1,3-butadiene (2j), 2,3-dimethyl-1,3-butadiene (2k), 1,2-dimethylenecyclohexane (2l)), and Ru(η5-C5Me4Et)(η4-diene)Br (diene = 2,4-hexadiene (3)) has been studied. 1,3-Butadiene and mono-, and 1,2-disubstituted-1,3-butadiene complexes afford bromo-substituted Ru(IV) anti η3-allyl complexes in high yields. This process involves addition of bromine on the exo face of the diene ligand taking place regioselectively at the terminal carbon bearing no substituent. Unexpectedly, bromination of 2d yields the dibromoruthenium(IV) η-(1−3)-hexa-1,4-dien-3-yl complex (6). In the course of this process HBr is liberated involving the intermediacy of bromo-substituted Ru(IV) η3-allyl complexes. The molecular structure of 6 has been determined. 1,4-Disubstituted-1,3-butadiene complexes 2f−h and 3 react with Br2 to form bromo-substituted Ru(IV) η3-allyl complexes 10a−c and 11 adopting exclusively the syn configuration. These compounds are not stable in solution and decompose to give either a dibromoruthenium(IV) η-(1−3)-hexa-1,4-dien-3-yl complex, as a result of HBr elimination, or the dimeric Ru(IV) complexes [Ru(η5-C5Me5)Br3]2 (13) and [Ru(η5-C5Me4Et)Br3]2 (14), respectively. In order to explain the observed stereochemistry and reactivity of complexes 10a−c and 11 a weak three-center 4e- C−Br···Ru interaction is proposed. In case of 2i, bromination leads to the formation of the complex salt [Ru(η5-C5Me5)(η6-C6H6)][Ru(η5-C5Me5)Br3] (15) and of the dimeric Ru(III) complex [Ru(η5-C5Me5)Br2]2. 15 features a novel monomeric 17e- half-sandwich Ru(III) complex as counteranion. The molecular structure of 15 has been determined. By contrast, bromination of 2,3-disubstituted-1,3-butadiene complexes 2j,k affords the novel cationic Ru(IV) η4-diene complexes [Ru(η5-C5Me5)(η4-diene)Br2]Br (diene = 2,3-dimethoxy-1,3-butadiene (16a), 2,3-dimethyl-1,3-butadiene (16b)). Complexes 16a,b are not stable in solution in the presence of Br-. On replacement of the bromide counterion by CF3SO3- the stable complexes 17a,b are obtained. The molecular structures of both 16a and 17a have been determined. Complexes 16 and 17 appear to be the first late transition-metal complexes approaching a σ2,π-metallacyclopentene structure.